Of the several types apparatus used in industry to indirectly exchange between fluids there are many situations where the straight tube type is the most desirable. In the straight tube type of heat exchanger one fluid passes through a straight tube of heat conductive metal having a wall thickness thin enough to permit the efficient exchange of heat between the fluid within the tube and the fluid flowing over it.
A straight tube heat exchanger can be cleaned and inspected more easily than one in which the heat exchange tubes are of configuration where the tubes are not straight. An eddy current device, for example, can be passed down a straight tube to allow for examination. Cleaning devices which could never be passed through a bent tube can quite easily be passed through a straight tube when it is desired to clean out the tubes during a major maintenance procedure. Moreover, in case a tube should fail, a straight tube can be plugged, by internal welding devices or explosive plugs.
Often the heat exchange fluid which is flowed through the heat exchange tubes is passed longitudinally to the lower portion of the heat exchanger through a centrally located supply conduit within the heat exchanger and then allowed to reverse direction and come back through the heat exchange tubes. For example, in heat exchangers where the fluid which supplies heat to the heat exchanger, that is the primary fluid is liquid sodium, the sodium preferably passes downward over the tubes and it is often required that the secondary fluid passes longitudinally of the tubes in the direction of the primary sodium through a centrally located supply pipe into a chamber below a tube sheet where the secondary fluid reverses its direction to flow back through the heat exchange tubes. Sudden changes in temperature of either the secondary or primary fluids can cause the supply pipe and tubes to contract or expand at different rates thus overstressing the tubes. The rate of expansion or contraction due to a sudden change in the temperature of the fluids passing through the supply pipe and heat exchange tubes will cause the heat exchange tubes to change in length faster than the supply pipe because of the thin walls of the heat exchange tubes. This causes the supply pipe to exert a force on the tubes which could lead to tube failures.